If you have been involved in the restoration of British cars, you have more than likely been exposed to the term ” taxable horsepower rating” . This has often been a source of confusion and amusement in the minds of North Americans, who are accustomed to large horsepower numbers and who haven’t had a tradition of petrol shortages and intrusive government policy towards their automobiles.
If you have looked at the spec sheet for your Singer Nine (pre or post war), you have likely noticed that although your car is identified as a “9” horsepower, it actually generates considerably more power (31 b.h.p. in the case of the Sports Nines of the 30’s) and perhaps sufficient horsepower to enable you to cruise the local freeway. So what is this lower horsepower rating anyway?
In 1921, the British government established the Road Act, which placed a tax on motors of one pound per RAC horsepower rating. In true bureaucratic form, however, the government wasn’t able to accept the manufacturers’ ratings and hence, devised their own standardized formula.
And a formula it was, not being based on any practical bench test, but relying on applied theory only. The formula that was devised was D2 N/2.5, where D represented the diameter of the cylinder in inches and N represented the number of cylinders. For example, let’s look at the Singer “9”. It has a four cylinder engine with 60mm pistons (2.3622 inches). The calculation to obtain the RAC Taxable horsepower rating would be as follows:
2.5
The figure 8.93 of course is rounded off to 9 to arrive at the taxable horsepower rating. However, if you look carefully at the calculation ( and recognize that it is the combined activity of the bore and stroke that develops horsepower), you will notice that the stroke does not enter into the calculation at all. Hence a very narrow bore, of 60mm for example, could be complemented with a long stroke that would increase horsepower well beyond that which is indicated by the government’s formula.
This formula fortunately worked in favour of the public, as well as the auto manufacturers of the day, as 1 pound for each horse of a low taxable rating was considerably less than the same pound multiplied by the much higher actual b.h.p.rating.